1. Field of the Invention
The present invention relates to separable adhesively bonded elements, to articles and assemblies including such separable adhesively bonded elements and to a method for separating such adhesively bonded elements from articles and assemblies. More particularly, the present invention relates to articles and assemblies having adhesively bonded shape memory alloy elements, to articles and assemblies including separable adhesively bonded elements and adhesively bonded shape memory alloy elements, and to a method of bonding and easily separating adhesively bonded elements, such as external components, employing shape memory alloy elements.
2. Brief Description of Related Art
In many industries, notably those related to the construction and/or manufacture of structures and devices in which later partial disassembly is required or anticipated, elements must be securely joined or affixed to one another for extended periods of time and still be capable of being easily separated without damaging or destroying one or more of the elements at the time of disassembly. This is of particular interest in certain aspects of the aircraft and automotive industries and in military vehicular applications, such as both wheeled and tracked ground vehicles and munitions carriers. In the construction of both fixed wing and rotary wing aircraft, such as helicopters, a particular concern is the attachment and removal of access panels and skin panels. Traditionally, such panels have been attached using mechanical fasteners. However, in modem aircraft such panels are generally constructed from advanced composite materials to reduce weight and radar cross-section. These composite materials are typically formed from fiber-reinforced polymers, such as glass or graphite fibers in an epoxy matrix. Tests have demonstrated that placing holes in composites, as is necessary when using mechanical fasteners, causes a significant loss in fatigue and static mechanical properties of the composites. Mechanical fasteners also increase the radar cross-section of the aircraft.
In order to avoid these problems, adhesives have frequently been employed to join such elements. These adhesives are designed to achieve high strengths and thereby provide necessary load transfer between the adhered elements. However, in some cases, the bonded panels need to be removed in order to repair or replace the panels, or to gain access to underlying components. For this reason, procedures are needed for the subsequent separation and disassembly of the adhered elements.
Currently, most structures having adhesively joined elements or components are debonded mechanically. This process typically includes physical prying, chiseling, grinding, sanding, and scraping of the adhesive and/or adhered element. This approach is slow and inconsistent, requires skilled technicians, may be environmentally hazardous both to the technicians and to the aircraft itself in releasing airborne particulates, and in many cases results in significant damage to the separated components or underlying structural members or skin. Damage to the separated components is particularly undesirable during removal of access panels, since it is preferred to reuse or reattach the panel after removal. If the panel is damaged during removal, then repair or replacement of the panel incurs additional time, cost, and logistical problems.
To avoid such problems, other variations of the foregoing adhesive bonding/debonding method have been tried. Most of these have involved a selective choice of adhesive employed and a specialized treatment of the adhesive in the debonding stage of the process. Examples of those adhesives (and the concomitant processes) that have been tried or considered have included hot-melt adhesives, solvent-specific removable adhesives, thermally removable adhesives, electrically debondable adhesives, and pressure-sensitive adhesives.
Hot-melt, or meltable, thermoplastic adhesives are weakened by heating the adhesive above the melt or glass transition temperature of the adhesive. However, only a partial loss in adhesive strength is achieved, so that significant mechanical force must also be applied to separate the adhered elements (adherends). Furthermore, separation must be achieved while the adhesive is hot since bond strength resumes upon cooling.
Solvent-specific-removable adhesives can be dissolved using solvents (including, inter alia, water). These adhesives, however, by nature, offer limited solvent resistance, which restricts their environmental and service durability. Adhesives that are formulated to dissolve in only strong solvents also present environmental concerns during adhesive removal.
Thermally removable adhesives have been developed which undergo a dramatic loss in mechanical properties at a given temperature due to a thermally initiated chemical reaction within the adhesive. Unlike hot-melt adhesives, a complete loss in mechanical strength occurs at the transformation temperature. Only limited performance data is available, so the mechanical strength and environmental durability of these adhesives is not known.
Electrically debondable adhesives have been developed which debond by passing a small electrical current (xcx9c1 mA) of low voltage (xcx9c10 V) through specially formulated epoxy adhesives to weaken and ultimately debond the adhesive. Typically, electrodes are embedded at the bondline with electrical leads to permit local voltage application. The fabrication, placement, and maintenance of these electrodes often present significant cost, design, and logistical burdens. Additionally, these adhesives currently offer low to moderate structural strengths, with significant loss in mechanical properties at elevated temperatures. The long-term durability of these adhesives is also not well-characterized.
Pressure-sensitive adhesives are low strength, removable adhesives. Their mechanical properties are not sufficient for most structural applications.
An additional limitation of all of these removable adhesives is that the debonding properties are enabled through specially designed adhesive chemistries. For many critical applications, especially in the aircraft industry, any new adhesive must be fully certified before it can be implemented in the final design. This certification process, including full environmental testing, is typically very expensive and requires long test durations. These factors often make implementation of these novel adhesives impractical.
Accordingly, it is an object of the invention to provide a more expeditious, facile and predictable means for affixing and later removing components than is currently available. To this end, it is also an object of the present invention to provide an adhesive bonding system and a method employing the adhesive bonding system to adhesively and securely join or bond at least two elements or components to one another with high bond strength and subsequently, optionally, and under preselected, specified conditions, debond and separate the bonded elements or components by weakening the strength of the adhesive.
It is also an object of the present invention to provide, particularly to vehicular means including, but not limited to, ground vehicles, both wheeled and tracked; aircraft, both fixed wing and rotary; and watercraft, an adhesive bonding system employing currently existing adhesives, and a more expeditious, facile and predictable method than is currently available of adhesively bonding at least two elements to one another with high bond strength and subsequently easily separating the adhesively bonded elements, using the adhesive bonding system under predetermined conditions.
It is another object of the present invention to provide an adhesive bonding system and a method of adhesively bonding at least two elements to one another with high bond strength and subsequently separating the adhesively bonded elements with little or no damage to the separated elements such that the elements may be subsequently rebonded.
It is a further object of the present invention to provide an adhesive bonding system and a method of adhesively bonding at least two elements to one another with a high bond strength and subsequently easily separating the adhesively bonded elements with few if any modifications required to the existing design, manufacturing, or repair specifications of either the elements undergoing bonding or the equipment or materials employed in the bonding process, other than the additional inclusion of shape memory alloy elements.
It is still another object of the present invention to provide an adhesive bonding system and a method of adhesively bonding at least two elements to one another with high bond strength and subsequently separating the adhesively bonded elements, which bonding system and method are both affordable and practical to implement.
It is an object of the present invention to provide an adhesive bonding system and a method of adhesively bonding at least two elements to one another with high bond strength and subsequently separating the adhesively bonded elements requiring minimal technical skill.
It is an object of the present invention to provide an adhesive bonding system and a method of adhesively bonding at least two elements to one another with high bond strength and easily subsequently separating the adhesively bonded elements with a minimum of environmental disturbance of an audible, chemical or particulate nature.
Further objects of the present invention include providing the aforementioned objects in an aeronautical environment, particularly in applications to aircraft of both fixed and rotary wing design.
This invention is suitable for use in any situation where it is desirable to adhesively and securely join or bond at least two elements or components and subsequently, optionally, and under pre-selected, specified conditions, debond, by weakening the strength of the adhesive, and separate the elements or components. Although generally applicable to most structures where it may be optionally desirable to subsequently remove a component part, the invention is particularly applicable to vehicular means including, but not limited to, ground vehicles, both wheeled and tracked; aircraft, both fixed wing and rotary; and watercraft.
The present invention further provides an adhesive bonding system and a method of adhesively bonding at least two elements to one another by forming an adhesive bond therebetween with a high bond strength, comparable or equivalent to bond strengths obtained with a conventional adhesive used in a conventional direct element-to-element adhesive bond configuration for the same type of bond, and subsequently weakening the adhesive bond formed between the adhesively bonded elements under predetermined controlled conditions such that the adhesively bonded elements may be debonded and easily separated.
The present invention additionally provides an adhesive bonding system and a method of adhesively bonding at least two elements to one another by forming an adhesive bond therebetween with a high bond strength and subsequently separating the adhesively bonded elements under predetermined controlled conditions with little or no damage to the separated elements such that one or more of the elements may be subsequently rebonded.
The present invention further provides an adhesive bonding system and a method of adhesively bonding at least two elements to one another by forming an adhesive bond therebetween with a high bond strength and subsequently easily separating the adhesively bonded elements under predetermined controlled conditions with little or no modifications required to the existing design, manufacturing, or repair specifications of either the elements undergoing bonding, to the equipment employed in the bonding process, or to the adhesive itself and wherein the bond strength and other properties of the cured adhesive prior to debonding are maintained under a variety of environmental conditions, including, but not limited to, elevated temperatures and high moisture conditions.
The present invention described herein also provides an adhesive bonding system and a method of adhesively bonding at least two elements to one another by forming an adhesive bond therebetween with a high bond strength and subsequently separating the adhesively bonded elements under predetermined controlled conditions, which bonding system and method are both affordable and practical to implement and involve a minimum of environmental disturbance of an audible, chemical or particulate nature.
The present invention also provides an adhesive bonding system and a method of adhesively bonding at least two elements to one another by forming an adhesive bond therebetween with a high bond strength and subsequently separating the adhesively bonded elements under predetermined controlled conditions, which method requires minimal technical skill or training to perform the procedures involved in assembly or disassembly of the component parts.
As used herein, the terms xe2x80x9cbond,xe2x80x9d xe2x80x9cbonding,xe2x80x9d or other variants thereof, have the usual meaning as the term is typically applied to adhesives, i.e., the setting or curing of a layer of adhesive between elements such that significant force is required to separate the elements. The term xe2x80x9cdebond,xe2x80x9d xe2x80x9cdebonding,xe2x80x9d or other variants thereof, as used herein, refer to an action or process which significantly or completely severs or weakens the bond formed between elements, such that the elements can be completely or substantially separated from one another with little force. The term xe2x80x9cbondline,xe2x80x9d as used herein, refers to the adhesive layer between bonded elements, as well as the interfaces of the bonded elements directly in contact with the adhesive. For the present invention, the term xe2x80x9cadherendxe2x80x9d refers to an element that is adhesively bonded or adhered, exclusive of the shape memory alloy (hereinafter also referred to as xe2x80x9cSMAxe2x80x9d) element. As used herein, the term xe2x80x9cSMA-to-adherend bondline,xe2x80x9d refers specifically to a bondline comprising a single adhesive layer between an SMA element and an adherend, as well as the associated SMA and adherend interfaces, while the term xe2x80x9coverall bondline,xe2x80x9d as used herein, refers to all SMA-to-adherend bondlines, as well as the associated SMA and adherend interfaces, associated with a single SMA element and the adherends to which that SMA element is bonded. As used herein, xe2x80x9cbondline failurexe2x80x9d refers to the condition where significant or complete loss of strength occurs anywhere within the overall bondline. xe2x80x9cAdhesive failure,xe2x80x9d as used herein, refers to a significant or complete loss in strength at the interface between an adhesive layer and the bonded adherend or SMA element. As used herein, xe2x80x9ccohesive failurexe2x80x9d refers to a significant or complete loss in strength within the adhesive layer, within the adherend, or within the SMA element. xe2x80x9cStiffness,xe2x80x9d as used herein, refers to the elastic modulus of a substance or material, independent of material geometry. xe2x80x9cRigidity,xe2x80x9d as used herein, refers to the general resistance of an element to any form of bending, straining, or deformation, and includes the effect of both material stiffness and geometry. The term xe2x80x9crigidxe2x80x9d is used herein to describe an adherend having high rigidity, such that it is difficult to bend, strain, or deform the adherend.
The invention described herein is capable of providing the aforementioned characteristics and advantages to most structures where it may be optionally desirable to subsequently easily remove a component part from the structure without damage to either the component part or the remainder of the structure. However, this invention is most useful in structural applications, such as in an aeronautical environment, particularly in applications to aircraft of both fixed and rotary wing design, and most particularly to the latter.
The aforementioned advantages and characteristics of the invention are attributed to a bonding/debonding system including at least one shape memory alloy element sandwiched or disposed between and adhesively bonded to facing surfaces of two opposed, rigid components, or adherends, the strength, stiffness and thickness of each of the adherends and adhesive used in the adhesive bonding being such that actuation of the shape memory alloy element causes debonding of the adhesive. Typically the facing surfaces of the opposing, rigid elements are substantially parallel to each other. Although depending somewhat on the shape of the SMA elements employed, typically each surface of an SMA element that is adhesively bonded to a surface of an opposing, rigid element is arranged in facing relationship and substantially parallel to the surface of the opposing, rigid element to which it is bonded.
The present invention is also directed to a method of adhesively bonding and selectively debonding a plurality of elements or components that includes mechanically stressing a shape memory alloy element to a mechanically strained or deformed state; sandwiching or disposing the strained shape memory alloy element between and adhesively bonding the strained shape memory alloy element to facing surfaces of two opposing, rigid elements; subsequently elevating the temperature of the deformed shape memory alloy element sufficiently to transform it from the mechanically strained state to a mechanically unstrained or non-deformed state, thereby weakening the adhesive bond formed between a surface of the shape memory alloy element and a facing surface of at least one of the two opposing, rigid elements such that the bond is severed; and separating the facing surfaces of two opposing elements.
While some of the embodiments of the present invention are described herein in terms relating to vehicular or aeronautical environments, it is to be understood that this invention is not limited solely to such applications, but is applicable to all situations in which a first rigid element is adhesively bonded to a second rigid element with the intention of subsequent separation of the elements without damage thereto.